The invention relates to a method for compacting moulding sand in a moulding box in which a model is present, which model is embedded in moulding sand, wherein the moulding box is vibrated by means of imbalance weights, which are each to be rotated about an axis of rotation during operation.
The term imbalance weight is understood to mean a mass to be rotated about an axis of rotation by means of a suitable driving source, whereby the center of gravity of said mass is located some distance away from the axis of rotation in question.
As is described in U.S. Pat. No. 4,600,046, such a method is used in particular when using foam models, which evaporate when molten metal is poured into the moulding box, whereby the vapors being formed will escape through the compacted moulding sand.
With the construction described in U.S. Pat. No. 4,600,046, the moulding box is supported on a spring-supported platform. A vibrating unit, which comprises two imbalance weights rotating in opposite directions during operation, is connected to the moulding box at a point located some distance above the springing platform. The two imbalance weights rotating in opposite directions generate a horizontally oriented vibration force during operation, which can only be varied in magnitude by increasing the frequency or by shifting the imbalance weights relative to each other during standstill of the device, but as a result of the spring support of the moulding box by means of the springing platform, it will not be possible to prevent the moulding box from making a tumbling movement as well during operation. Furthermore, when the counterweights are rotating, a vibration force only in substantially horizontal direction will constantly be generated.
According to the invention, at least four imbalance weights are used, which can be shifted in phase relative to each other.
When using the method according to the invention, vibration in vertical direction as well as in horizontal direction can be generated as desired, whereby also the magnitude of the vibration force can be varied independently of the frequency of the movement of the imbalance weights. Preferably, this will be done in such a manner that the resulting vibration force will pass through the center of gravity of the moulding box. Furthermore it is possible to arrange that no vibration forces will be exerted on the moulding box when the imbalance weights are rotating, which makes it possible in an advantageous manner to run the imbalance weights up to the desired speed first, and only then produce a vibration force by means of the imbalance weights. It is possible thereby to avoid vibration of the moulding box at a frequency corresponding with the natural vibration frequency of the moulding box during the starting and stopping of the rotation of the imbalance weights, whereby undesirable movements affecting the intended compacting of the moulding sand may occur.
According to another aspect of the invention, a device which is particularly suitable for carrying out the above-described method comprises a frame for supporting a moulding box, whilst the frame supports four imbalance weights which are rotatable about axes of rotation extending parallel to each other, and means are provided for driving the imbalance weights and shifting them in phase relative to each other.